The invention relates to a circuitry which improves performance of a low cost electret microphone.
Electret microphones are preferred in many applications for their small size, light weight and high output. They are manufactured in numerous acoustic configurations, but generally fall in one of three categories in terms of electronic circuitry: two-terminal simple, three-terminal intermediate and three-terminal advanced. The electret element is enclosed in a housing together with a buffer amplifier, most commonly a JFET stage. A voltage bias source is therefore required for the JFET amplifier and is provided by the equipment the microphone is connected to.
The two-terminal simple electret mic is a modest, low cost device suitable for consumer electronic appliances and telephones, where any limitations in acoustic to electrical conversion are masked by bandwidth shaping (often mandated by FCC rules) or the ability of a human listener to comprehend speech with moderate distortion. The mic housing contains the absolute minimum necessary for operation, an electret diaphragm and a JFET. One side of the charged electret is grounded to the case, the opposite side is internally coupled to the JFET gate. The source is brought outside as one of the two terminals but is grounded to the case. The other terminal is the JFET drain. A typical two-terminal electret mic is the Panasonic WM-54BT.
The three-terminal advanced electret mic has additional circuitry, typically resistors and sometimes also capacitors mounted on a hybrid circuit board together with the electret and the FET. The greater complexity and cost are offset by higher performance, particularly greater intelligibility and lower distortion. Applications for these microphones are in speech recognition, hearing aids, military communications, and music. FIG. 1 is a schematic diagram of a three terminal electret mic 100. The mic 100 includes a grounded housing 102, an electret diaphragm element 104 and a JFET transistor 106. The mic has three terminals: ground 108, FET source 110 and drain 112 enclosed in the grounded housing. Most commonly, the drain is directly connected to the bias voltage source or battery and the JFET 106 is used as a source follower for low output impedance. Typical ranges for biasing resistors 114 and 116 are several megohms and several hundred ohms respectively. Gentex Model 3065 is a representative advanced electret microphone.
The intermediate three-terminal electret is simpler than the advanced by avoiding the hybrid circuit board but still brings out the JFET source connection separately. Primo EM-60P is such a microphone.